The present invention relates to field accessed, cylindrical domain bubble memories and, more specifically, to a control circuit for the rotating magnetic field.
Magnetic bubble domains constitute a type of domain structure characteristic of thin films of magnetic materials which possess a uniaxial magnetic arisotrophy with the easy direction of magnetization perpendicular to the plane of the film. A domain which is enclosed by a single continuous domain wall shrinks to the shape of a cylindrical magnetic bubble when a magnetic field bias of a particular magnetic is applied perpendicular to the plane of the film. The magnetic domain bubbles are mobile and move toward a region of lower bias field in a gradient. Various methods have been devised for producing localized and variable gradients in the bias field to give coherent bubble motion or propagation. The most generally useful method of that in which an overlay pattern of thin film, soft magnetic elements is placed close to the bubble film and excited with a rotating magnetic field in the plane of the film. For a given orientation of the rotating field, the resulting magnetic polarization of these elements produces fringing fields which act to reduce the bias field in some regions and to increase it in others; the bubbles always move to the regions of low bias field. As the in-plane field rotates, these fringing fields vary with the varying polarization of the permalloy element and can be used to produce coherent motion of the bubble.
The circuit for creating the rotating magnetic field commonly includes a pair of coils which are positioned perpendicular to each other and an AC current source for supplying current to the coils. A separate current is supplied to each coil, and the two AC currents are 90.degree. out of phase. The total effect of the individual magnetic fields from the current in each coil is a rotating magnetic field vector which extends throughout a region into which the bubble memory is disposed.
One of the important requirements of this system is the need for the rotating magnetic field to be coherent so that the data storage remains in a non-volatile state. In other words, any particular bubble, which represents a bit of digital data, must have a predictable location within the propagation track at all times. Thus, the currents which produce the rotating field during the operate mode must have the proper phase relationship, frequency and amplitude. The location of the bubbles must also be predictable during the start and end of the operate mode. If there is no current in the coils when the AC current is initiated or if the current goes to zero at the end of the operate mode, a time delay is introduced to the inductive nature of the coils. This delay could cause the loss of coherent operation of the memory during the beginning and at the end of the operate mode.
Prior art is known which deals with minimizing the tendency of transient conditions from causing the loss of non-volatile data storage in bubble memories. The U.S. Pat. No. 3,763,478 describes various control circuits for the currents which create the rotating magnetic field. In one circuit, a DC current is supplied to the coils at the beginning and at the end of the application of AC current to create a static field vector condition. The DC current is less than the peak AC value. The control circuit described in this patent includes a pair of switches in series with the coil. It has been found that switches produce distortion in the AC current. This distortion may interfere with the signals generated by the bubble detector circuits. To the knowledge of the present inventors, no control circuit is known which reduces the time for energizing the coils to a minimum at the beginning and end of the rotating magnetic field in a economical distortion free manner.